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Although Sn-Pb eutectic alloy is widely used as a joining material in the electronics industry, it has well documented environmental and toxicity issues. Sandia National Laboratories is developing alternative solder materials to replace traditional Pb-containing alloys. The alloys are based on the Sn-Ag, Sn-Ag-Bi and Sn-Ag-Bi-Au systems. Prototype hybrid microcircuit (HMC) test vehicles have been developed to evaluate these Pb-free solders, using Au-Pt-Pd thick film metallization. Populated test vehicles with surface mount devices have been designed and fabricated to evaluate the reliability of surface mount solder joints. The test components consist of a variety of dummy chip capacitors and leadless ceramic chip carriers (LCCC's). Intermetallic compound (IMC) layer reaction products that form at the solder/substrate interface have been characterized and their respective growth kinetics quantified. Thicker IMC layers pose a potential reliability problem with solder joint integrity. Since the IMC layer is brittle, the likelihood of mechanical failure of a joint in service is increased. The effect of microstructure and the response of these different materials to wetting, aging and mechanical testing was also investigated. Solid-state reaction data for intermetallic formation and mechanical properties of the solder joints are reported.
This thesis presents a series of mechanical test methods and comprehensively investigates the deformation and damage behavior of Cu/Pb-free solder joints under different loading conditions. The fracture behavior of Pb-free joint interfaces induced by stress, deformation of solder and substrate are shown, the shear fracture strength of the Cu6Sn5 IMC is measured experimentally for the first time, and the dynamic damage process and microstructure evolution behavior of Pb-free solder joints are revealed intuitively. The thesis puts forward the argument that the local cumulative damage is the major cause of failure in solder joints. The research results provide the experimental and theoretical basis for improving the reliability of solder joints.
The technology drivers of the electronics industry continue to be systems miniaturization and reliability, in addition to addressing a variety of important environmental issues. Although the Sn-Pb eutectic alloy is widely used as a joining material in the electronics industry, it has drawn environmental concern due to its Pb content. The solder acts both as an electrical and mechanical connection within the different packaging levels in an electronic device. New Pb-free solders are being developed at Sandia National Laboratories. The alloys are based on the Sn-Ag alloy, having Bi and Au additions. Prototype hybrid microcircuit (HMC) test vehicles have been assembled to evaluate Pb-free solders for Au-Pt-Pd thick film soldering. The test components consist of a variety of dummy chip capacitors and leadless ceramic chip carriers (LCCC's). The mechanical properties of the joints were evaluated. The reflow profiles and the solid state intermetallic formation reaction will also be presented. Improved solder joint manufacturability and increased fatigue resistance solder alloys are the goals of these materials.
Based on the results of a more than two-year study, Lead-Free Electronics: iNEMI Projects Lead to Successful Manufacturing is the first practical, primary reference to cover Pb-free solder assembly as well as the analysis and reasoning behind the selection of Sn-Ag-Cu as the recommended Pb-free replacement for Sn-Pb. Reflecting the results of a two-year study, Lead-Free Electronics: iNEMI Projects Lead to Successful Manufacturing provides full coverage of the issues surrounding the implementation of Pb-free solder into electronic board assembly. This book is extremely timelyâ??most electronic manufacturers are going to change over to Pb free soldering by 2006 to meet new European laws. All manufacturers around the globe are going to be affected by this change. The text provides specific results from the thirty company NEMI project activities. It contains integrated and fully documented book chapters with references to existing published work in the area. These serve as tremendous resources for engineers and companies faced with making the switch to Pb-free solder assembly.
Even though the effect of lead contamination on human health has been known for decades, very little attention has been paid to lead-based solders used in electronics until recently. This comprehensive book examines all the important issues associated with lead-free electronic solder. It collects the work of researchers recognized for their significant scientific contributions in the area.
Continuous miniaturization of microelectronic interconnects demands smaller joints with comparable microstructural and structural sizes. As the size of joints become smaller, the volume of intermetallics (IMCs) becomes comparable with the joint size. As a result, the kinetics of bond formation changes and the types and thicknesses of IMC phases that form within the constrained region of the bond varies. Furthermore, the size of the grains becomes comparable with the size of the bond and the bond may only consist of a few grains resulting in an anisotropic behavior. The effect of size of solder joints on mechanical behavior is not clearly understood and contradicting results have been reported. Studies have shown that some size effects introduced strengthening effects while others have shown weakening effects associated with decreasing joint size. While numerous studies have been dedicated to evaluating elastic and plastic properties of a variety of solder alloys in bulk and large-scale (several 100s of microns) joint configurations, very few studies have been directed to address the elastic and plastic properties of joints in the scale of few microns. This dissertation focuses on investigating combination effects of process parameters and size on kinetics of bond formation, resulting microstructure and the mechanical properties of joints that are formed under structurally constrained conditions. An experiment is designed where several process parameters such as time of bonding, temperature, and pressure, and bond thickness as structural chracteristic, are varied at multiple levels. The experiment is then implemented on the process. Scanning electron microscope (SEM) is then utilized to determine the bond thickness, IMC phases and their thicknesses, and morphology of the bonds. Electron backscatter diffraction (EBSD) is used to determine the grain size in different regions, including the bulk solder, and different IMC phases. Physics-based analytical models have been developed for growth kinetics of IMC compounds and are verified using the experimental results. Nanoindentation is used to determine the mechanical behavior of IMC phases in joints in different scales. Four-point bending notched multilayer specimen and four-point bending technique were used to determine fracture toughness of the bonds containing IMCs. Analytical modeling of peeling and shear stresses and fracture toughness in tri-layer four-point bend specimen containing intermetallic layer was developed and was verified and validated using finite element simulation and experimental results. The experiment is used in conjunction with the model to calculate and verify the fracture toughness of Cu6Sn5 IMC materials. As expected two different IMC phases, [nu]-phase (Cu6Sn5) and [epsilon]-phase (Cu3Sn), were found in almost all the cases regardless of the process parameters and size levels. The physics-based analytical model was successfully able to capture the governing mechanisms of IMC growth: chemical reaction controlled and diffusion-controlled. Examination of microstructures of solder joints of different sizes revealed the size of the solder joint has no effect on the type of IMCs formed during the process. Joint size, however, affected the thickness of IMC layers significantly. IMC layers formed in the solder joints of smaller sizes were found to be thicker than those in the solder joints of larger sizes. The growth rate constants and activation energies of Cu3Sn IMC layer were also reported and related to joint thickness. In an effort to optimize the EBSD imaging in the multi-layer configuration, an improved specimen preparation technique and optimum software parameters were determined. Nanoindentation results show that size effects play a major role on the mechanical properties of micro-scale solder joints. Smaller joints show higher Young's modulus, hardness, and yield strength and lower work hardening exponents comparing to thicker joints. To obtain the stress concentration factors in a multilayer specimen with IMC layer as bonding material, a four-point bending notched configuration was used. The analytical solutions developed for peeling and shear stresses in notched structure were used to evaluate the stresses at IMC interface layers. Results were in good agreement with the finite-element simulation. The values of interfacial stresses were utilized in obtaining fracture toughness of the IMC material.
Examination of the bulk and interface (solder/substrate) microstructures by optical microscopy, scanning electron microscopy and energy dispersive spectroscopy was performed. This was done, in part, to compare research microstructures with industrial standards in order to determine appropriate cooling rates and evolution of second phases with and without transition metal additions. Evaluations of shear strength, tensile strength, and Vickers micro-hardness, on drawn wire samples and solder joints of as-solidified and annealed specimen, showed the effect of transition metal additions to the Sn-Ag-Cu based alloys. It is clear from the results to date that the addition of cobalt, nickel or antimony to Sn-Ag-Cu near eutectic solder improves both the elevated temperature microstructural stability, strength, and hardness above that for Sn-40Pb, Sn-3.5AG, Sn-0.7Ag or Sn-4.7Ag-1.7Cu.